As shown in FIG. 1, a cellular wireless communication system is mainly composed of a mobile terminal, a wireless access network, and a core network.
For the cellular wireless communication system, the mobile terminal needs to send a random access message via uplink common channel before communicating with the network. Such a common channel is referred to as random access channel. The main purpose of random access is to occupy limited wireless channel resources in a competitive manner, and/or to obtain uplink synchronous information. The uplink synchronous information is utilized by the mobile terminal to determine when to send uplink signal, so that the uplink signal falls into the signal reception time window of a base station (BS) receiver.
The channels of the cellular wireless communication system can be distinguished by all possible technologies, such as code division, frequency division, and time division, and the minimum unit in time domain of each channel is a frame. The frame or frame combination for random access is a random access time slot, and the random access time slots of the same channel compose a random access sub-channel in a certain way. In FIG. 2, 3 frequency carriers (FCs) and 3 channel codes (CCs) compose 9 channels, for example, the FC 2 and the CC 2 compose a channel, and one of every 6 frames on this channel is utilized as a random access time slot which form a random access sub-channel on this channel, and the spacing between two random access time slots are 5 frames. The channel composed of FC 1 and CC 1 is also configured with a random access sub-channel, and thus there are 2 random access sub-channels in this cell.
The message per se employs a very low data rate in order to ensure an effective coverage of an initial random access message, in other words, the message has limited information bits. For instance, in the LTE (Long Term Evolution) system of currently discussed 3GPP (3rd Generation Partnership Project), a random time slot composed of non-synchronous single frames (duration: 1 ms) can carry information not more than 10 bits.
Generally, the information bits of a random access message are represented by sequence number of character codes with high self-correlation. During one random access time slot, the wireless access network can distinguish random access messages containing various character codes, and thus a collision occurs only when two random access messages with an identical character code are received during a same random access time slot. The collision will result in that the wireless access network can not distinguish the individual terminals, that is to say, in the view of the wireless access network, the effect is the same as that only access request from one terminal is received. In this circumstance, an additional collision resolution mechanism between the terminals and the wireless access network is required to ensure that one of the terminals can access successfully, while the failed ones will access to the wireless network in such competitive manner again. Sometimes, a collision can result in failure in the access of all terminals involved in the collision.
The initial access starting from a free state (idle state) is one reason for random access, and one more important reason is the handoff access between BSs.
During the communication between a terminal and a network, when the terminal moves among different cells, the wireless channel needs to be switched to keep the communication. When a target cell and a source cell belong to different BSs, one possible handoff manner is that a target BS has provided wireless channel resources during the handoff preparation stage, and then informs the terminal to access to the wireless channel allocated by the target BS via a source BS. This handoff manner is very effective for systems based on dedicated wireless channel resource, such as GSM (Global System for Mobile Communication), but it will increase the system complexity and thus result in a low utilization ratio of the wireless channel resource for wireless communication systems based on shared channel, such as LTE, this is because the resource BS needs to know the time difference between the source BS and the target BS at the beginning of the handoff so as to inform the terminal of a proper time to access to the target BS. However, it is very difficult to obtain a precise time difference, and therefore the wireless access network needs to employ a complex mechanism to acquire the time difference. Besides, because the transmission time of a message in the wireless access network or wireless interface is uncertain, it is very difficult for the target BS to reserve shared wireless channel resources, a too early reservation will lower the utilization efficiency of the wireless channel, and a too late reservation will result in missing access time of the terminal or introducing an additional delay, and therefore some redundant wireless channel resources will be reserved to ensure successful handoff, which will lower the utilization efficiency of the wireless resources. So generally, a handoff manner without reserving wireless channel resources will be employed, that is to say, the terminals acquire wireless channel resources at the target BS in a competitive random access manner. With the technological development of cellular wireless communication system, the time for such random access procedure is very short, for example, it takes about 20 ms in the LTE system.
When a terminal being in communication moves among cells, means such as handoff need to be employed to keep the communication continuity. Generally, channels except common channels employ channel share mechanism, and the uplink and downlink wireless resources are scheduled by wireless access network.
The delay for a terminal in accessing to wireless access network via random access is very important to either access reason. However, the delay of handoff access, i.e. the delay due to handoff interruption is subject to a stricter requirement by the cellular wireless communication system. When a terminal moves among cells, the most primary and direct reason for handoff is that the signal strength of the original cell has decreased to such a dangerous extent that the communication quality of the terminal will be deteriorated, even phenomena such as call drop and interruption will occur, if the wireless channel is not switched to a cell with better signal quality. For a terminal, there is obvious difference between the introduction in free state or inactive state and the introduction in service active state for a same delay, and the terminal is more sensitive to the latter. During a random access procedure, the delay is mostly incurred from the above said “collision”, because the collision will result in that only one terminal can access to the wireless network, or even all terminals fail in the access, and even if the failed ones can re-access successfully, that will result in more delay. It is necessary to provide a method to eliminate the collision probability during the handoff access procedure of a terminal based on the importance of handoff access.
Eliminating the collision probability during handoff access can be realized by reserving dedicated random access character codes. However, in some peculiar circumstances, for example, there are not enough reserved character codes, the target BS will not be able to allocate reserved character codes, thus leading to handoff failure.
Thus, there is a need for a handoff access solution for cellular wireless communication system to solve above mentioned problems.